In the manufacture of sputter targets used in the semiconductor industry, and more particularly sputter targets used in physical vapor deposition (PVD) of thin films onto complex integrated circuits, it is desirable to produce a sputter target that will provide film uniformity, minimal particle generation during sputtering, desired electrical properties and minimal chip defect. Furthermore, to meet the reliability requirements for diffusion barriers or plugs of complex integrated circuits, the sputter target must have high-purity and high-density.
Cu/Cr thin films are commonly used as the adhesion/barrier layer in flip-chip bonding to form the interconnection between the semiconductor chip and the substrate, such as an aluminum bonding pad or ceramic base. The Cu/Cr film is deposited between a Cu layer and a Cr layer. The Cu layer adheres the solder, typically 90Pb/10Sn, to the chip, whereas the Cr bonds to the Al pad or ceramic base.
Current methods to achieve suitable Cu/Cr sputter targets for use in complex integrated circuits involve blending fine particle size Cu and Cr powders followed by sintering the powder by hot-pressing or hot-isostatic-pressing. Using either of these techniques to press powders with powder sizes less than 20 .mu.m, the density of the pressed target material is about 90% of theoretical density. The fine grain size of the powder materials is believed to contribute to higher uniformity of the deposited film. However, the sputter targets fabricated by these methods have a high oxygen content, on the order of about 3000 ppm average. The inventors of the present invention have discovered that the fine powder size distribution used in Cu/Cr targets is a large contributing factor to the high oxygen content in the pressed sputter target blanks, and that oxygen in the Cu/Cr layer contributed via target sputtering is detrimental to the integrity of the film, and results in an increase in the percentage of chip defect. The sputter targets currently produced, which have an average oxygen content of about 3000 ppm, generate defects in about 3% of the semiconductor chips, on average. Thus, the Cu/Cr sputter targets fabricated by hot-pressing or hot-isostatic-pressing fine particle size powders have proved unreliable for use in complex integrated circuits.
There is thus a need to develop a method for fabricating low-oxygen, high-density Cu/Cr sputter targets that will meet the reliability requirements for complex integrated circuits, and specifically, that will decrease or eliminate chip defects due to high oxygen content.